1. Field of the Invention
The invention relates to a method for producing metallic formed articles with a ceramic layer by the membrane method, wherein a porous metallic membrane is used, a metallic formed article with a ceramic layer, and the use of such a metallic formed article.
2. Description of the Related Art
DE 195 24 750 A1 describes the production of a ceramic layer on the surface of a ceramic formed article, by applying powder particles on a metallic membrane by electrophoretic deposition. A stabilized suspension is used for this purpose.
DE 195 20 458 A1 describes a device for the electrophoretic coating of substrates with two main electrodes to produce a homogeneous electric field and means of arrangement for positioning a substrate within the homogeneous electrical region of the electric field. There are also means of homogenizing the electric field, for example auxiliary electrodes that counteract the distortion of the electric field by the substrate.
U.S. Pat. No. 5,002,647 A discloses a method for producing thick films, in which a powdered starting material is introduced into solvent and an electrical potential is then applied between the electrodes placed in the solution, whereby the powder is deposited on a substrate connected to the cathode. A special solvent system is suggested for this method.
JP 2001316874 A describes a corresponding method in which an electrode is placed in the interior of a nonconducting porous substrate, so that the substrate is coated when a potential is applied.
However, these publications are concerned with conventional electrophoretic deposition, while in the present case, on the other hand, electrophoretic deposition occurs by the membrane method as disclosed by EP 0 446 999 B1.
In this case, in contrast to conventional electrophoretic deposition, a membrane is placed between anode and cathode to separate physically the pH change in the vicinity of the electrodes caused by H+ and OH− ions from the place of deposition. The evolution of gas that takes place because of the electrolytic dissociation of water can also not affect the particle motion and deposition because of this spatial separation.
Porous metallic formed articles, because of their high stability accompanied by low densities, are used in many industrial sectors. Examples that may be mentioned are filter membranes, structural elements in light construction, implant materials, and anodes for solid oxide fuel cells (SOFCs).
In many cases it is necessary or of great benefit to introduce a second phase (for example, a ceramic phase) that provides catalytic properties, biocompatibility, or other physical characteristics (ion conduction in particular), and/or enables better adhesion to the surrounding material.
A gradual transition between the materials with different thermal expansions is desirable in temperature-stressed material composites (for example, the anode-electrolyte layer in the SOFCs), in order to avoid stresses and cracking. An additional ceramic component is necessary for using metallic formed articles as anodes in solid oxide fuel cells. It makes ion conduction possible in the used volume of the anode and improves the adhesion of the electrolyte layer.
There are two types of anodes that differ in principle: Supporting anodes carry the entire 3-layer system of the SOFC and are between 300 μm and 1,500 μm thick. Nonsupporting anodes such as those described in EP 0 829 103 B1, for example, are thinner, about 50 μm.
Three different configurations of the microstructure are known. Either a mixture of the ceramic and metallic particles is used (for example, EP 0 525 844 B1). However, anodes consisting of a simple mixture of ceramic and metal have numerous drawbacks: First, one is bound to percolation theory and the necessary porosity of 30% can only just be achieved. Furthermore, this configuration tends toward superpotential losses. Also, the metal is not protected against corrosion in this configuration.
It is possible alternatively to configure the microstructure by partially enveloping or coating a ceramic framework with metal, or coating a metallic framework with a ceramic. These two alternatives are preferred over a mixture of ceramic and metallic particles, but are much more costly to produce. An important advantage of the coated metallic framework consists in the fact that the metal is protected against corrosive attack, which for some time has been held responsible as the reason for the degradation and failure of SOFC layers.
The following production methods are known for this:                1. Preparation of supporting anodes by pressing or hot-pressing. Microstructure: mixture.        2. Preparation of supporting anodes by thermal spray methods, particularly plasma spraying. Microstructure: mixture.        3. Application of the anodes as nickel slips on the electrolyte layer. A very tedious and cost-intensive impregnation of the anode with YSZ follows, by electrochemical gas phase deposition, EVD, at high temperatures. The anodes made in this way achieve the highest efficiency in the present state of the art. Microstructure: metal framework with ceramic coating.        4. Anode preparation by film printing. Microstructure: mixture.        5. Construction of anodes as a mixture of two phases.        6. Impregnation of sintered nickel membranes with stabilized YSZ suspension by saturation, as shown in EP 0 439 938 B1.        
The high costs and the considerable time required, the toxic gases formed, and poor controllability are drawbacks in the 3rd method. A graduated coating cannot be produced by this method.
The 6th method requires two sintering steps. Only low green densities can be achieved by saturation.